What’s So Great About x4?

x4 has come full circle. Announced in 2006, x4 made headlines as a technological breakthrough.

By 2007, x4 was generally viewed as a problematic disappointment.

Here we are in late 2008 and x4 is back in fashion, once again being trumpeted as a technological breakthrough. Curiously, the company putting x4 back in the headlines is none other than Samsung, SanDisk’s arch-rival, and ardent suitor.

Samsung has even put a value on x4- “billions of dollars.”

From SanDisk’s recent arbitration win over Samsung:

“According to the testimony at the hearings, an early termination of the license caused by a termination of the Strategic Agreement will have serious financial consequences. This is due to M-Systems announcement in the second quarter of 2006 of a technological breakthrough called x4 technology, which enables, for the first time, the use of 4 bits per cell NAND. This effectively doubles the capacity of the 2 bits per cell NAND technology now utilized in flash memory products. According to the testimony, this new technology has significant implications for personal computers and all entertainment devices. The timing of M-Systems Notice of Termination did not escape Samsung who argues strenuously that, without the license granted under the Strategic Agreement, it will only be able to acquire the rights to the new x4 technology through a renewal or extension of its existing technology cross-license with SanDisk. According to Samsung, SanDisk will use the leverage gained from a victory in this arbitration to force additional royalties that could potentially cost Samsung billions of dollars.”

So What’s So Great About x4?

There’s more to x4 than meets the eye. Most folks seem to think of x4 simply as a slightly bulked up version of 3 bit/cell MLC (x3.) If it were that simple, Samsung would never have uttered “billions.”

When msystems announced x4, some experts in the business came right out and said msystems claims of 4 bits/cell performance and reliability simply weren’t possible.

In fact, a slightly bulked up version of x3 probably isn’t workable. Too squirrelly.

msystems succeeded at the so-called impossible 4 bits/cell by coming at the problem from an entirely different angle. x4 isn’t a straightforward implementation of 4 bits/cell. Its another beast altogether.  How it works has never been revealed, but I’ve got my guesses.

Before speculating further, some background and back-story are in order.

Background and Back-Story

As discussed, SanDisk comes by x4 via its acquisition of msystems in July 2006.

x4 was announced on msystems one and only analyst day in May 2006. This msystems analyst day was webcast live. Luckily I recorded the event. No replay was ever available

These presentations include x4 info that to my knowledge has never been presented elsewhere. The presentations were done in two parts, I’ve posted transcriptions of both. Part I has most of the good stuff. Particularly recommend Dov’s comments, Simon Litsyn’s presentation and the Q&A:

2006.05.11 msystems Analyst Day Part I transcript

My post “What happened to x4?” attempts to update the x4 saga within the context of July 2007. In hindsight, this post still holds up, but misses the x4 alternative reality (joke).

My post “Stratosphere and x4”, in many respects, is the inspiration for this post. The most important thing to remember when reading this post is that my basic thesis is wrong. x4 is a subset of Stratosphere. The real deal is revealed in Amir Ban’s comments following the post. As he put it:

“Interesting and intelligent analysis, but the 7,023,735 patent is not the “x4 patent”. Indeed, a search for an “x4 patent” must come up empty-handed, since x4 is a straightforward application of Stratosphere. Stratosphere envisions taking some baseline flash and increasing bits/cell, using methods described in 6,469,931 and later patents. x4 means using the same methods when the baseline is 2 bit/cell MLC and the target is 4 bit/cell.”

Many thanks Amir.

x4 technology

x4 is, in many respects, a misnomer. The fundamental technology isn’t 4 bit/cell specific. The original name for the technology was Stratosphere. To my knowledge this name is no longer used. x4 came later. As Amir put it:

“The term x4 was coined by my Stratosphere group in a meeting with Toshiba in March 2004 to describe the specific cooperation proposal we were putting forward to them.”

While it is true that x4 is a technology that when applied to regular MLC (2 bits/cell) doubles storage capacity to 4 bits/cell, it can be also be applied to SLC resulting in x1.58±.  or can be applied to x3 resulting in x5± (possibly).

It can also be used to improve NAND reliability, and isn’t necessarily limited to flash memory.

The key x4 technology patent appears to be U.S. patent #6,469,931, “Method for Increasing Information Content in a Computer Memory.”  This patent was applied for in 2001 and issued in 2002. Inventors are listed as Amir Ban, Simon Litsyn and Idan Alrod. Assignee is msystems.

The key idea is that groups of flash memory cells can store information above and beyond the information stored in the flash memory cells themselves. With a few twists, a top-up in memory capacity is possible for “free.” The image below, from the patent, illustrates a schematic description of a two-dimensional optimal encoding scheme.

One example given in the patent is for single level cell flash memory.  Today, 16 cells of SLC NAND can store 16 bits of information.  Using x4 technology, 20 bits of information can be stored using the same 16 cells of SLC- a 25% gain in capacity.

Capacity gains increase when more memory cells are used, reaching a limit of around 58% for SLC. Hence x4 technology can theoretically turn x1 (SLC) into x1.58.

Capacity gains increase as the bits/cell increase as well. When x4 technology is applied to 2 bits/cell MLC, 4 bits of information can be stored. Hence the name x4.

Theoretically x4 can be applied to 3 bits/cell resulting in something close to x5, but this may not be practical as x3 already uses aggressive coding and the voltage levels may not be stable.

A second fundamental key x4 technology patent appears to be U.S. patent #7,023,735, “Method for Increasing the Reliability of a Flash Memory.”  This patent was applied for in 2004 and issued in 2006. Inventors are listed as Amir Ban, Simon Litsyn and Idan Alrod. Assignee is Ramot at Tel-Aviv University.

The main idea is that by programming flash memory using reference voltages, fractional reference voltages, and redundancy bits, the reliability of info stored in MLC flash memory can be improved. Again, just like patent #6,469,931, each memory cell is not considered only by itself. Reliability can be improved by taking advantage of the frame of reference offered by groups of memory cells.

I can’t help but wonder if SanDisk hasn’t found a use for x4 technology beyond the the dedicated 4 bit/cell chip. Clearly the technology itself would seem to lend itself to many applications. The first which comes to mind is MLC SSDs- Not 4 bit/cell MLC, but rather 2 bit/cell SSDs with improved performance and reliability. That be sweet- if improvements were significant.

This might help explain Samsung’s contention that x4 will prove important enough to cost Samsung billions of dollars in licensing fees. If its something else that’s OK too. Inquiring minds want to know.

SanDisk’s lawyers may yet figure out a way to bring Samsung’s testimony public. Maybe then we’ll have a better idea of why Samsung thinks x4 is so great.


4 Responses to What’s So Great About x4?

  1. Poofypuppy says:

    Thanks, Savo, for shedding renewed light on x4. I’m not smart enough to understand the mathematical algorithms and everything else behind x4, but was wondering if my layman’s understanding below is correct…

    (1) x4 technology is the combination of MLC (2 bits per cell) NAND flash and patented software algorithms that utilize groups of cells to “create” additional storage bits.

    (2) This (almost) doubling algorithm is implemented using a separate and special controller chip. This controller chip is manufactured by/at Tower Semiconductor (Israel).

    My question is why x4 (despite having been patented years ago and discussed in previous conference calls) has not reached production yet. Is SanDisk waiting on 43nm production to ramp up before putting it into production? Is there a problem securing enough fab capacity to make the needed controllers? Is there a problem with algorithm performance (i.e. too slow)? I guess I’m just curious what is keeping x4 from entering widespread production (manufacture and sale).


  2. Mrs. Cencomco says:

    Thanks for the excellent post. You have a way of translating very complicated info into simple terms, and I very much appreciate that.
    To add to Poofy’s question, from what I have read in various posts, Samsung’s license apparently encompasses all IP, which in this case, includes X4 from Dec. 2006 when SanDisk acquired M-Systems. I would think that this has a lot to do with the delay in implementing X4, to possibly Aug. 2009 without a resign. Appreciate your comments.
    Mrs. Cencomco

  3. savolainen says:

    Greetings Poofy,

    x4, the 4/bit cell variety, is a combination of a special chip, special controller and special software. Not much is known about the chip itself, but from what I can gather the chip would be 2 bits/cell MLC with the addition of special circuitry (in the chip) surrounding the NAND transistors. Dov made a big point that no special equipment would be needed for fabrication.

    I don’t know where the x4 controllers will be fabricated. It has never been announced to my knowledge. For a while I suspected TSEM would be manufacturing some, but now I’m not so sure. When Eli stepped down from the TSEM board, I took it as a negative signal.

    x4 is scheduled to be introduced in 2009 @ 43nm. I have pretty low expectations for the performance of the initial production. SanDisk seems to have low expectations as well. Initial x4 is targeting A/V applications which are particularly forgiving.  As time goes by x4 performance will probably improve- just like the performance of 2 bits/cell MLC has improved.

    After the FLSH acquisition, SNDK pushed the x4 release back from FLSH’s plans.

    There are probably several reasons why. It probably made more sense to just push really hard on shrinking geometries. This was going to be most profitable.

    The x4 technology itself could have needed tuning for mass production.

    The consumer market probably didn’t need the larger capacities that x4 enables.

    Then there are the legal angles as Mrs. Cencomco mentioned. Before pushing forward, it probably made sense to have this arbitration resolved.

    In general x4 should become more important with each geometry shrink from here on out. Each shrink will take longer and cost more. The wall is probably 2x nanometers, but you never know. If x4 fulfills its promise, better to hit the competition when there is maximum leverage.

    All this said, x4 still has to prove itself. The delay could just be because x4 isn’t up to the task. Time will tell.


  4. Poofypuppy says:

    Hi Savo,

    Thanks for your additional thoughts. I’m trying to mentally reconcile the purported billion-dollar value of the x4 IP (to Samsung) with its actual performance and practicality. There don’t seem to be enough details for the average-Joe investor to judge, but that’s nothing new. Am wondering also how 3D cost/performance potential compares to x4. Perhaps SanDisk is sitting back, watering both trees and waiting to see which one will ultimately bear more fruit (while also waiting for any legal/IP storms to pass, as you and Mrs. Cencomco noted). Even though SanDisk is sitting on large amounts of cash, I tend to doubt it has enough resources to fully invest in and successfully bring both technologies (x4 and 3D) to market at the same time. Would be interesting to see SanDisk’s latest road map for these.

    But I guess that’s what makes technology investing (and SanDisk in particular) such an interesting game. Thanks again for all your insights.


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